SEVERE REDUCTION OF RAT DEFENSIVE BEHAVIOR TO A PREDATOR BY DISCRETE HYPOTHALAMIC CHEMICAL LESIONS

Nonspecific lesion and stimulation methods have suggested that the hyp othalamus is critical for the expression of defensive behavior, althou gh the organization of neural circuits mediating such behavior is uncl ear. In the rat hypothalamus, we found that increased Fos levels were restricted to specific cell groups following presentation of a stimulu s (predator) known to elicit partly innate defensive responses. The do rsal premammillary nucleus showed the most striking increase in Fos le vels, and cell body-specific chemical lesions therein virtually elimin ated two major components of defensive behavior but increased explorat ory behavior, suggesting that this caudal hypothalamic nucleus plays a critical role in the expression of behavioral responses sometimes cri tical for survival of the individual. We have previously shown that th e Fos-responsive cell groups in the medial hypothalamus are interconne cted in a neural system distinct from those mediating reproductive and ingestive behaviors. (C) 1997 Elsevier Science Inc.